- #3140
- clone `MediaTypeHeaderValue` instance before updating it when content negotiation succeeds
- avoids changes to `MediaTypeConstants` properties and `OutputFormatter.SupportedMediaTypes` entries
- `MediaTypeHeaderValue.Clone()` does not exist in our DNX Core fork of this class
- in previous implementation, was called defensively rather than when required
- update `WebApiCompatShimBasicTest` functional tests to use `MvcTestFixture<TStartup>` everywhere
- #3140 blocked that final migration
- remove `TestHelper` since it's no longer referenced
nits:
- remove comments mentioning `TestHelper`
- correct spelling of "negotiation"
53060be2d7